The world places conflicting demands on our brains. When we are intentionally paying attention to sense impressions or tasks, “stimulus-independent-thought” (aka “tasked-unrelated-thought” aka “mind wandering”) is unable to proceed. This is because focusing on something taps into the same general cognitive resource, one which can only “handle only one coherent data stream at a time” (Teasdale et al 1995, p 38). In other words, paying attention to what we’re doing or what we’re perceiving disrupts the progression of thought and vice versa. To focus on one stream of data prevents us from focusing on other streams of data. So if we’re looking at a painting and also thinking about the painting, these cognitive acts are alternating. The switching back and forth often happens so quickly that we’re unaware that one stream has been turned off while another becomes active. It may feel like a seamless experience but it’s not. There’s a lot of switching going on.

Of course, our brains are constantly processing multiple streams of information and making decisions outside of what we happen to be paying attention to. We couldn’t walk, drive, eat, talk, or basically do anything without the automatic brain system doing all that simultaneous information processing. That type of processing is happening in outside of and along the edges of consciousness. It’s not the same as paying attention.

So, can we be “present” to the external world while watching our thoughts “unfold” in real time. I’m thinking not. Different data streams are involved. At best, we can flit from stream to stream, making it seem simultaneous. But it’s not.


Teasdale, J. D., Segal, Z. V., & Williams, J. M. G. (1995). How does cognitive therapy prevent depressive relapse and why should attentional control (mindfulness) training help? Behaviour Research and Therapy, 33, 25-39.

Teasdale, J. D., Dritschell, B. H., Taylor, M. J., Proctor, L., Lloyd, C. A., Nimmo-Smith, I., et al. (1995). Stimulus-independent-thought depends upon central executive resources. Memory and Cognition, 28, 551–559.